Firearms have conventionally been aimed by placing them manually at a fixed distance from the firer in the line of sight between his eye and an intended target. The traditional method has been to line up a backsight with a foresight and set the combined image against the intended target. A single line of vision towards the target was thus established, providing the sighting was done with one eye. The second eye, being in a different position, observed the two sights as being completely out of line with the target. It was therefore normally kept closed. This gave a restricted field of vision. Moreover, since the eye can only focus on one distance at a time and with open sights there are three planes, i.e. target, foresight and backsight, of necessity two are blurred. An improvement was made with the introduction of an apertured backsight because it was found that the eye will automatically centre itself in the centre of a small round aperture which is in close proximity to the eye, leaving only the foresight and target to be considered. In this case, the eye must focus on the foresight leaving the target blurred giving greater accuracy than if the target is focussed.
To improve accuracy still further, firearms have been provided with telescopic sights whereby a firer can observe an enlarged image of a target through the sight. Aiming was normally achieved by means of cross wires or similar markings on the sight. While considerable accuracy can be achieved with such sights, a disadvantage is that only a limited field of fire can be viewed through the sight. Furthermore, while under poor light conditions a target may be easily distinguished with natural binocular vision, to find the target with a telescopic sight is very difficult if not impossible. The difficulty increases with increased magnification. Thus, while such sights are eminently suitable for stationary easily-identified targets, they are not easy to use with moving or snap targets.
Further, since the eye which is not applied to the sight is usually closed, the firer's vision is even more restricted than when using so-called open sights consisting of a foresight and a backsight on the firearm.
Vision using one eye is unnatural. The human brain is equipped to provide us with binocular vision, whereby the conflicting visual information it receives from two separate sources--our eyes--is computed into a single image. In most people, one eye tends to be more dominant than the other and is often called the "master" eye. It is this eye which we keep open when shooting in the traditional way. Right-handed people normally have a dominant right eye and left-handed people a dominant left eye. With one eye closed, however, our field of vision is markedly reduced and in combat situations this can be a fatal restriction.
It has previously been proposed to overcome the disadvantage of conventional open sights and telescopic sights by providing the sighting device which forms the subject of British Patent Specification No. 1332512. The sighting device according to this prior patent comprised a housing and an outer tube receiving the housing and adapted to be mounted on a firearm. The angular position of the housing in the outer tube was adjustable and the tube had a rearward opening to be looked into by a firer when aiming the firearm. The housing carried a light unit comprising a light collecting member of luminous material having a surface for exposure to ambient light and arranged to emit light in a rearward direction. An aperture in the housing rearward of the light unit provided a circular opening to light emitted from the light unit whereby a luminous spot was visible through the rearward opening of the outer tube. The outer tube was so mounted on the firearm that when the firearm was pointed at a target the luminous spot viewed through the rearward opening in the outer tube coincided with the aiming point of the target.
With such a sighting device, a firer can aim the firearm at a target while keeping both eyes open. One eye will observe the whole field of fire while the other eye, which is applied to the rear end of the sighting device, will see the luminous spot as well as the target area which is not obscured by the sighting device. As a result, the firer's brain will in effect see a luminous spot superimposed on the field of fire. When the spot coincides with a desired aiming point, the firer can fire his weapon secure in the knowledge that his firearm is correctly aimed at the target.
This prior sighting device is easier to use than the previously known sights and does not suffer from their disadvantages. In particular, both eyes are kept open so that the firer has an unrestricted field of view of a target area and can observe and hit moving targets more easily. The device does, however, still suffer from a number of disadvantages. In particular, the provision of an outer tube makes the device cumbersome and adjustment of the housing within the tube is not easy. Further, the light emitted by the light collecting member is visible at night thus making the firer vulnerable to enemy attack.